Cancer authorities recognize that perhaps second only to smoking cessation, the choice of a diet containing fresh fruits and vegetables is the best strategy to avoid cancer. While this has become a truism, and some nutritional chemicals have been shown to have anti-cancer properties, little is known about the mechanisms by which dietary nutrients may prevent the appearance of cancer. However, beginning nearly 50 years ago, it was shown that certain chemicals, among them compounds such as isothiocyanates (ITCs), prevent carcinogen-induced cancers in animals. The prevailing model is that ITCs transcriptionally increase expression of carcinogen-detoxifying enzymes, termed Phase 2 genes. We and others propose a different model in which a major feature of chemoprevention is the instigation of an apoptotic cell death response in incipient tumor cells. The ability of dietary ITCs to induce apoptotic cell death and to alter signal transduction cascades acutely supports this model. Molecular mechanisms that explain the anti-cancer activities of ITCs are lacking. While electrophiles like ITCs could covalently modify proteins, the ability of ITCs to covalently label target effector proteins has not been explored. We developed a novel method to detect and purify proteins covalently modified by chemopreventive isothiocyanates, either in vitro, in cells, or even in intact animals. We have shown that perhaps surprisingly, ITCs covalently modify very few proteins within the cell, and we have identified a single protein that represents over 90% of the covalent ITC-modified protein within human tumor cells. This protein has been implicated in several types of human cancers;for example it is highly overexpressed in cancers of colonic and non-small cell lung origin. This target molecule is also known to control transcriptional responses dependent on the API transcription response element, suggesting a mechanism through which it may control expression of Phase 2 genes, and corroborating its likely role as a target of cancer chemoprevention. We propose to use knockout animals deficient in this target to further characterize the role of this protein in cancer chemoprevention, and to develop it as a biomarker useful for quantifying the effect of ITC chemopreventives. In addition to in vivo carcinogenesis experiments, we will confirm previous results that cells from knockout animals lacking this ITC target are more difficult to transform into tumor cells, and identify specific mechanisms that could explain chemoprevention by ITCs.